Project summary A haplotype is a sequence of alleles on a chromosome. Two haplotypes are identical by descent if they are inherited from a common ancestor. Segments of identity by descent (IBD) are found at high frequency in population samples and can be detected with high accuracy in high density SNP array or sequence data. IBD segments can be used for computationally efficient and accurate estimation in many health-related applications including estimating population genetic parameters and performing genetic association testing. We propose to develop improved methodology for detecting IBD segments, with particular focus on quantifying uncertainty in IBD segment endpoints. We will also develop methods that use IBD segments to estimate mutation rates, migration rates, effective population sizes, and population split times.